Recent advances in pacemaker technology have rapidly introduced new clinical applications. The therapeutic benefit of pacing for all kinds of cardiac arrhythmias has been significantly increased by means of multiprogrammability and physiologic pacing.
Since a wide range of pacing parameters are programmable, pacemaker function can be closely tailored to the patient's needs in order to optimize the haemodynamic benefit. Pacing therapy has also been introduced for tachycardia control. Contemporary designs for implantable devices also include diagnostic features. Pacemaker patients come to follow-up sessions in a pacemaker center according to a prescribed individual schedule. An important purpose of these sessions is to either diagnose or to prevent the occurrence of events that are hazardous to the patient. A hazardous event can occur due to an impending pacing system failure, a cardiac pacing malfunction or disturbance, or a cardiac arrhythmia. Contemporary follow-up is based on accurate electrocardiologic and electronic measurements as well as on radiographic imaging.
The most frequent hazardous pacing system failure is battery depletion. While quite rare, lead fracture and insulation brakes may also occur. Various kinds of oversensing and undersensing phenomena are the most common pacing disturbances. The most hazardous pacing malfunction is loss of capture which may be the consequence of exit block, i.e. a rise in threshold above the programmed output. Lifethreatening arrhythmias can be triggered by premature ventricular contractions which are not always felt by the patient. Since pacemaker mediated tachycardia can also appear to be slow, the patient does not always feel this event. Many complications occur transiently, while certain physiologic and pathophysiologic conditions are sustained. Since all of the possible complications cannot be detected by the patient and his physician, it is possible for a problem to remain unidentified for extended periods of time.
During recent years of pacemaker design development, there have been many efforts to provide various detectors and indicators of pacing complications, in order to enhance safety by providing a warning to the patient and the physician.
Generally, the most efficient follow-up procedures utilize features incorporated into the pacemaker itself. Non-programmable pacemakers generally include only a magnet test function. The test frequency (of pacing) depends on the battery voltage, and decreases whenever the battery voltage decreases. There are also pacemakers, such as described in U.S. Pat. No. 3,842,844, which include electronic circuits which increase output pulse width as battery voltage decreases. There are programmable pacemakers which can be programmed after implantation by means of an external programmer. Pacemaker programming is a basic feature of patient follow-up. The underlying rhythm can be observed using low frequency programming. Overdrive of the spontaneous rhythm can be accomplished by increasing pacing frequency.
Since a rise of threshold precedes some pacing malfunctions, it is essential to determine the safety margin of the pacing output. As described in U.S. Pat. No. 3,713,449, the threshold can be measured by successive decreases of the output pulse width.
The pacing mode, the sensitivity, the refractory period and many other parameters can be programmed in multiprogrammable pacemakers. Precise programming within the optimal range of parameters can prevent many complications. Some complications can be intentionally provoked during a follow-up session by programming to particular combinations of parameters. This is especially significant in dual-chamber pacemakers.
A major advance in follow-up procedures was obtained by the introduction of telemetric pacemakers into clinical practice. Interrogation and telemetry readout are performed by using the programmer and provide many significant data values, such as, for example, values of the current program. If the pacemaker includes measurement functions, the telemetry readout can also provide values of the battery voltage, internal impedance of the battery, the measured lead impedance and the values of measured output parameters.
Recent pacemaker development is directed towards autoprogrammability. Many pacemakers include a backup mode of pacing which maintains stimulation in the event of significant battery depletion or microprocessor failure. Some pacemakers have an auto-capture function. The latter have electronic circuits for evoked response detection and the output is automatically increased whenever loss of capture is detected. If the threshold does not exceed the maximum output, the automatic output programming prevents complications caused by exit block. The newest dual-chamber and antitachycardia devices also include some important diagnostic features. Event counters memorize the number of premature ventricular contractions (PVCs) or the number of upper rate excesses. Bradycardia counters and interference counters are also used in follow-up. Antitachycardia pacemakers can provide a telemetry readout which indicates the number of tachycardia attacks, the number of successful and failed terminations, as well as the number of termination attempts.
Despite development in follow-up methods, patient safety depends largely on the follow-up interval schedule. In order to increase patient safety, many inventors have proposed electronic circuits for pacemakers which include special functions for patient control and warning. The basic clinical principle is to provide some signal to the patient whenever a pacing malfunction or a pacing system failure occurs.
One prior art example is an implantable device comprising an auxiliary battery which is switched on as a backup power supply whenever the primary power source is significantly depleted. An indication of switching is provided either by a variation of the pulse rate or by an additional set of electrodes remote from the heart. An acoustic signal has also been proposed for warning of impending battery depletion in pacemakers. A transducer or piezoelectric crystal for generating acoustic signals upon battery end-of-life detection is used. This type of design consumes additional energy for sound generation. Since sound is significantly attentuated within subcutaneous tissue, the efficiency of these systems is limited by the obesity of patients. Furthermore, slight deafness as well as the patient's ignorance of the signal cannot be excluded as important application considerations.
Instead of using acoustic signals, it is more convenient to provoke a non-hazardous symptom, easily recognizable by the patient. The most efficient alarm symptom is muscle twitching. It is actually a common complication of high output pacing in the unipolar mode. Since it does not affect pacing efficiency, it is not hazardous. However, pain tolerance is a psychological problem. A unipolar pacing system has an indifferent electrode (anode) on the pacemaker can. A bipolar system has an indifferent electrode on the lead located proximally to the active electrode. Programmable polarity systems can switch the indifferent electrodes in order to obtain either bipolar or unipolar pacing.
Since muscle twitching is provoked by a high strength electric field within the muscle, the majority of unipolar pacemakers use a partially insulated can in a manner which directs the electric field of the indifferent electrode toward the skin. A thin insulation layer covering the can is interrupted to leave a relatively small uninsulated area on one side of the can which serves as the indifferent electrode.
A body tissue stimulation apparatus with a warning device has been proposed in U.S. Pat. No. 4,140,131. An implantable pacemaker comprising a battery voltage level detector as well as a lead impedance level detector is disclosed. If the voltage level falls below a predetermined limit or the lead impedance changes so that it is outside a predetermined range, the warning device will be activated. The warning device includes a special output circuit which is isolated from the pacing output circuit and an auxiliary electrode (cathode). Bipolar warning stimulation of the muscle is performed through the auxiliary electrode and the indifferent electrode (pacemaker can). Different rates of the warning pulse train are used to indicate battery depletion and lead failure respectively. In the disclosed embodiment, the additional output circuit drains additional power from the battery. If the auxiliary electrode is surrounded by the indifferent electrode (as disclosed), it may be fixed on the pacemaker can. A consequence is that pacemaker can production is a more complex undertaking. Insulation and sealing between the auxiliary and indifferent electrodes as well as a more complex design of the pacemaker can are required.
It is also desirable to provide an alarm to a patient if failures other than battery and lead malfunction occur or if cardiac arrhythmias occur. It is not important that the patient be given enough information to diagnose the nature of the malfunction. It is much more desirable that the patient be able to differentiate between non hazardous malfunctions or alarms given merely for diagnostic purposes, and alarms for hazardous malfunctions. It is very important that the pacemaker include a reliable mode of back-up or safety pacing.